The present invention relates to a charging and discharging control circuit including a delay circuit using a capacitor for a secondary battery, and a charging type power supply device using the charging and discharging control circuit.
A conventional charging and discharging control circuit including a delay circuit using a capacitor for a secondary battery, and a charging type power supply device using the charging and discharging control circuit is shown in FIG. 2. In general, the charging type power supply devices have been put on the market as battery packs used in notebook PCs, camera integrated VTRs, or the like.
An external charger is connected to external terminals 106 and 107 to charge a secondary battery 103. When the secondary battery 103 passes into an over-charge state after the charging has progressed, a comparator is inverted so that a delay circuit 102 starts to charge a capacitor 104. When the capacitor reaches a given voltage, the delay circuit 102 outputs a signal that turns a switch element 101 off to stop charging the secondary battery 103.
In the case of the delay circuit shown in FIG. 2, which uses the charge and discharge operation of the capacitor the device, suffers from the following problem.
In the delay circuit described above, when the capacitor 104 fails and becomes placed in a short-circuiting state, a terminal 108 connected with the capacitor 104 is always fixed to a ground potential.
In this situation, when the secondary battery 103 starts to be charged, and the secondary battery is over-charged, the capacitor 104 must start to be charged. However, because the capacitor 104 is in a state of failure, a voltage at the terminal 108 is at ground potential and does not rise. Accordingly, the delay circuit 102 does not output a signal that makes the switch element 101 turn off. As a result, the charging operation progresses, and the secondary battery 103 ignites in the case where it is, for example, a lithium ion secondary battery.
In general, in a circuit provided for protection or an improvement in safety, some protection must be provided even if failure occurs. That is, so-called "fail safe operation" must be carried out.